This invention relates to an improvement in the cell assembly used in the electrolytic deposition of metals such as zinc. In a conventional zinc recovery process, roasted zinc ore is leached with a sulfuric acid solution to dissolve the zinc and form a solution of zinc sulfate. The pure zinc is then recovered from the zinc sulfate solution by electrolysis, the zinc being deposited on the cathode plates which form part of a series circuit through the electrolyte to the anode plates. When the electrolysis is complete, the zinc is stripped from the plates and processed to prepare the bulk metal.
Since the beginning of the commercial use of electrolysis to recover zinc from zinc sulfate solutions aluminum cathodes have been used. The zinc is deposited on the aluminum cathode from the zinc sulfate solution and stripped off periodically for melting.
The use of aluminum cathodes poses several problems. Aluminum cathodes have a high initial cost due to the fact that a specially refined metal must be employed in their manufacture. Ordinary commercial aluminum is not satisfactory for cathode use due to its high tendency to corrode in the presence of an electrolyte. High-grade aluminum is also corroded by electrolytes commonly employed, especially above the solution line. In addition, the deposited zinc has a tendency to stick to the aluminum cathode due to the presence of fluoride ion in the electrolyte which etches the aluminum cathode. Furthermore, it is generally necessary to apply edge strips to facilitate stripping of the zinc deposits from the aluminum.
U.S. Pat. No. 2,058,259 discloses the substitution of zinc cathodes for the aluminum cathodes and suggests that the problems of solution line corrosion can be solved by attaching a film or coating of a protective material to the starting sheet prior to commencement of the electrolytic operation to effectively inhibit corrosion of the cathode at the solution line.
U.S. Pat. No. 3,579,431 relates to a specific cell design for the deposition of metal and incidentally discloses the application of a coating of material, preferably polyvinyl chloride resin approximately four inches wide, to the aluminum cathodes to provide a coating about two inches above and two inches below the solution line.